Plasma and brain kinetics of large neutral amino acids and of striatum monoamines in rats given aspartame

Alterations in behaviour, cerebral cortical morphology and cerebral oxidative stress markers following aspartame ingestion

OBJECTIVE

The study evaluated changes in open field behaviours, cerebral cortical histomorphology and biochemical markers of oxidative stress following repeated administration of aspartame in mice.

METHODOLOGY

Adult mice were assigned into five groups of twelve each. Vehicle (distilled water), or aspartame (20, 40, 80 and 160mg/kg body weight) were administered orally for 28days. Horizontal locomotion, rearing and grooming were assessed after the first and last dose of aspartame. Sections of the cerebral cortex were processed and stained for general histology, and also examined for neuritic plaques using the Bielschwosky's protocol. Glial fibrillary acidic protein (GFAP) and neuron specific enolase (NSE) immunoreactivity were assessed using appropriate antibodies. Aspartate and antioxidant levels were also assayed from cerebral cortex homogenates. Data obtained were analysed using descriptive and inferential statistics.

RESULTS

Body weight and food consumption decreased significantly with aspartame consumption. Locomotion, rearing and grooming increased significantly after first dose, and with repeated administration of aspartame. Histological changes consistent with neuronal damage were seen at 40, 80 and 160mg/kg. Neuritic plaque formation was not evident; while GFAP-reactive astrocytes and NSE-reactive neurons increased at 40 and 80mg/kg but decreased at 160mg/kg. Superoxide dismutase and nitric oxide increased with increasing doses of aspartame, while aspartate levels showed no significant difference.

CONCLUSION

The study showed morphological alterations consistent with neuronal injury and biochemical changes of oxidative stress. These data therefore supports the need for caution in the indiscriminate use of aspartame as a non-nutritive sweetener.

Low-calorie sweetener use and energy balance: Results from experimental studies in animals, and large-scale prospective studies in humans

For more than a decade, pioneering animal studies conducted by investigators at Purdue University have provided evidence to support a central thesis: that the uncoupling of sweet taste and caloric intake by low-calorie sweeteners (LCS) can disrupt an animal's ability to predict the metabolic consequences of sweet taste, and thereby impair the animal's ability to respond appropriately to sweet-tasting foods. These investigators' work has been replicated and extended internationally. There now exists a body of evidence, from a number of investigators, that animals chronically exposed to any of a range of LCSs - including saccharin, sucralose, acesulfame potassium, aspartame, or the combination of erythritol+aspartame - have exhibited one or more of the following conditions: increased food consumption, lower post-prandial thermogenesis, increased weight gain, greater percent body fat, decreased GLP-1 release during glucose tolerance testing, and significantly greater fasting glucose, glucose area under the curve during glucose tolerance testing, and hyperinsulinemia, compared with animals exposed to plain water or - in many cases - even to calorically-sweetened foods or liquids. Adverse impacts of LCS have appeared diminished in animals on dietary restriction, but were pronounced among males, animals genetically predisposed to obesity, and animals with diet-induced obesity. Impacts have been especially striking in animals on high-energy diets: diets high in fats and sugars, and diets which resemble a highly-processed 'Western' diet, including trans-fatty acids and monosodium glutamate. These studies have offered both support for, and biologically plausible mechanisms to explain, the results from a series of large-scale, long-term prospective observational studies conducted in humans, in which longitudinal increases in weight, abdominal adiposity, and incidence of overweight and obesity have been observed among study participants who reported using diet sodas and other LCS-sweetened beverages daily or more often at baseline. Furthermore, frequent use of diet beverages has been associated prospectively with increased long-term risk and/or hazard of a number of cardiometabolic conditions usually considered to be among the sequelae of obesity: hypertension, metabolic syndrome, diabetes, depression, kidney dysfunction, heart attack, stroke, and even cardiovascular and total mortality. Reverse causality does not appear to explain fully the increased risk observed across all of these studies, the majority of which have included key potential confounders as covariates. These have included body mass index or waist circumference at baseline; total caloric intake and specific macronutrient intake; physical activity; smoking; demographic and other relevant risk factors; and/or family history of disease. Whether non-LCS ingredients in diet beverages might have independently increased the weight gain and/or cardiometabolic risk observed among frequent consumers of LCS-sweetened beverages deserves further exploration. In the meantime, however, there is a striking congruence between results from animal research and a number of large-scale, long-term observational studies in humans, in finding significantly increased weight gain, adiposity, incidence of obesity, cardiometabolic risk, and even total mortality among individuals with chronic, daily exposure to low-calorie sweeteners - and these results are troubling.

Chronic Effect of Aspartame on Ionic Homeostasis and Monoamine Neurotransmitters in the Rat Brain

Aspartame is one of the most widely used artificial sweeteners globally. Data concerning acute neurotoxicity of aspartame is controversial, and knowledge on its chronic effect is limited. In the current study, we investigated the chronic effects of aspartame on ionic homeostasis and regional monoamine neurotransmitter concentrations in the brain. Our results showed that aspartame at high dose caused a disturbance in ionic homeostasis and induced apoptosis in the brain. We also investigated the effects of aspartame on brain regional monoamine synthesis, and the results revealed that there was a significant decrease of dopamine in corpus striatum and cerebral cortex and of serotonin in corpus striatum. Moreover, aspartame treatment significantly alters the tyrosine hydroxylase activity and amino acids levels in the brain. Our data suggest that chronic use of aspartame may affect electrolyte homeostasis and monoamine neurotransmitter synthesis dose dependently, and this might have a possible effect on cognitive functions.

Establishing the upper end of the range of adequate and safe intakes for amino acids: a toxicologist's viewpoint

The safety assessment of high intake levels of individual amino acids cannot be based on data from nutritional studies with proteins. Routine toxicity tests designed to investigate a wide range of possible effects should be undertaken for hazard identification and characterization using studies selected to mirror the predicted pattern and duration of human exposure. The approach used to establish an acceptable daily intake level for additives and pesticides, based on defining a "no observed adverse effect" level in the experimental study and dividing by uncertainty factors that allow for species differences and human variability, has a long history of use for foreign compounds and would provide a suitable basis for determining health-based guidance values for single amino acids. The usual default uncertainty factors for toxicokinetics and toxicodynamics should be replaced by compound-specific values if suitable data are available. In addition, the usual uncertainty factors should be modified to more relevant default values based on species differences and human variability in the biodisposition of amino acids in general or of groups of metabolically interrelated amino acids. There would be no significant health concerns if the human intake levels were below a health-based guidance value developed using this approach. A population-distribution approach could be used to define the magnitude of any risk at intake levels above the guidance value.

Aspartame: review of safety

Over 20 years have elapsed since aspartame was approved by regulatory agencies as a sweetener and flavor enhancer. The safety of aspartame and its metabolic constituents was established through extensive toxicology studies in laboratory animals, using much greater doses than people could possibly consume. Its safety was further confirmed through studies in several human subpopulations, including healthy infants, children, adolescents, and adults; obese individuals; diabetics; lactating women; and individuals heterozygous (PKUH) for the genetic disease phenylketonuria (PKU) who have a decreased ability to metabolize the essential amino acid, phenylalanine. Several scientific issues continued to be raised after approval, largely as a concern for theoretical toxicity from its metabolic components--the amino acids, aspartate and phenylalanine, and methanol--even though dietary exposure to these components is much greater than from aspartame. Nonetheless, additional research, including evaluations of possible associations between aspartame and headaches, seizures, behavior, cognition, and mood as well as allergic-type reactions and use by potentially sensitive subpopulations, has continued after approval. These findings are reviewed here. The safety testing of aspartame has gone well beyond that required to evaluate the safety of a food additive. When all the research on aspartame, including evaluations in both the premarketing and postmarketing periods, is examined as a whole, it is clear that aspartame is safe, and there are no unresolved questions regarding its safety under conditions of intended use.

Interspecies and interstrain studies on the increased susceptibility to metrazol-induced convulsions in animals given aspartame

The ability of aspartame (APM) to increase the susceptibility to metrazol-induced convulsions was studied in two strains of mice (CD1 and DBA/2J) and in guinea-pigs. Rats were included as known positive controls. Plasma and brain levels of phenylalanine (Phe) and tyrosine (Tyr) were measured in CD1 mice and guinea-pigs at various intervals after a dose of 1 g APM/kg body weight (administered orally to mice and ip to guinea-pigs). In mice, peak levels of Phe and Tyr were observed in plasma after 30 min and in brain after 60 min. In guinea-pigs peak plasma levels of Phe and Tyr occurred 30 min after treatment. Phe was at a maximum in guinea-pig brain after 30 min, while Tyr levels reached a peak at 120 min. In further experiments Phe and Tyr levels were measured 1 hr after APM doses of 0.5, 0.75 or 1 g/kg. In CD1 mice, plasma Phe and Tyr levels were increased significantly only at the highest dose, whereas in brain, Tyr concentrations were significantly increased by 0.75 or 1 g APM/kg and Phe was significantly increased by all three doses. In the guinea-pig, plasma Phe and Tyr were increased significantly only by 1 g APM/kg and in brain this dose significantly raised only the Phe levels. Monoamine and metabolite levels were determined in the brain striata of CD1 and DBA/2J mice 1 hr after the oral administration of 1 or 2 g APM/kg body weight; no differences from control values were found in either strain. The studies of potentiation of metrazol-induced convulsions showed that APM, at doses of up to 2 g/kg body weight, had no such effect in mice or guinea-pigs. In contrast, as expected, the potentiation was significant in the rat at 1 g/kg.